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Characterizing and Controlling Carbon Nanomaterials

Final Report Summary - CCCAN (Characterizing and Controlling Carbon Nanomaterials)

The project CCCAN was dedicated to a full characterization and control of carbon nanostructures, in particular graphene and carbon nanotubes, motivated by the need for high-quality, well controlled material for future nanoelectronic and optical applications. The project combined complementary methods, i.e. vibrational spectroscopy, scanning probe microscopy, and theoretical modeling to obtain a comprehensive understanding of the electronic, optical, and structural properties of these materials.
We implemented two novel methodologies for this project: tunable UV-Raman spectroscopy and tip-enhanced Raman scattering (TERS). With tunable UV-Raman spectroscopy, we gained access to higher-lying electronic transitions, in particular the pi-pi* transition in sp2-bonded carbon nanomaterials and the excited-state geometry of nanometer-sized molecules with sp2 (graphene-like) and sp3 (diamond-like) hybridized carbon. Tip-enhanced Raman scattering provides optical spectroscopy beyond the classical diffraction limit and thereby gives information about the local properties in nanostructures. Here we have further developed a special fabrication method for the metal tips, which allows for combined electrical and topological probe irrespective of the conductivity of the sample. This will give rise to new future research activities regarding the light-matter interaction in the near-field and the quantum mechanical description of the TERS process. Furthermore, we have performed benchmark experiments for our TERS setup, providing nanometer-scale hyperspectral imaging of InN clusters in InGaN nanostructures, and demonstrated the polarization-independence of TERS in carbon nanotubes.

Within the course of the project, we have contributed to the toolbox for carbon nanomaterials’ characterization with the following results: we identified a “new” Raman mode in few-layer graphene that counts the number of layers. Moreover, we have developed a generalized treatment of interlayer vibrational modes valid for any two-dimensional few-layer material, which is relevant not just for counting the number of layers, but in particular for analysis of layer-layer interaction.
In a combined theoretical and experimental approach, we have finally resolved a long-standing debate about the most prominent Raman mode in graphene and bilayer graphene, the so-called 2D mode, which had previously been incorrectly interpreted. These results are highly relevant for interpretation of polarization dependent Raman experiments in uniaxially strained graphene, which reveal the crystallographic orientation of graphene. Our DFT-based calculations and the experiments of the 2D mode in uniaxially strained graphene as well as all reports in literature support our revised interpretation.
Furthermore, we have shown that UV-Raman spectroscopy in graphite, graphene, and carbon nanotubes (with excitation energies above ~ 4.7 eV) gives Raman spectra “beyond the double resonance” without the common 2D mode. Instead, they show the 2nd-order two-phonon density of states, giving for the first time a clear measurement of the IR-active phonon modes.
In carbon nanotubes, we have revisited the double-resonant scattering of the defect-induced D mode for arbitrary nanotube structure [given by the chiral index (n,m)] by experiments on (n,m) identified/sorted nanotubes and numerical simulations of the Raman spectra. We have shown that the apparently contradicting reports in literature about the tube-diameter dependence of the D mode can be understood when taking the excitation energy into account. Second, we have shown how the effects from individual nanotubes and ensembles can be discriminated and how they contribute differently to the D mode. Our results imply that the D mode is indicative of specific (n,m) nanotubes and thus can be used for detecting (n,m) specific defects. This is relevant for proving (n,m) selective chemical functionalization.
With respect to interactions of carbon nanomaterials with their environment, we have investigated graphene in devices, graphene with intentionally introduced defects, laser-induced oxidation of graphene, and graphene from different growth methods (CVD, MBE) on different technologically relevant substrates.
Going beyond the initial scope of the project (starting from WP(4) about interactions with the environment), we have explored different routes for fabrication of heterostructures of graphene and other layered materials, developed a universal model for interlayer vibrations in few-layer materials, and investigated theoretically the indirect doping of atomically thin materials through doping of the underlying material.